Crate replace_with
source ·Expand description
Temporarily take ownership of a value at a mutable location, and replace it with a new value based on the old one.
This crate provides the function replace_with()
, which is like std::mem::replace()
except it allows the replacement value to be mapped from the original value.
See RFC 1736 for a lot of discussion as to its
merits. It was never merged, and the desired ability to temporarily move out of &mut T
doesn’t
exist yet, so this crate is my interim solution.
It’s very akin to take_mut
, though uses Drop
instead of
std::panic::catch_unwind()
to react to unwinding, which avoids the optimisation barrier of
calling the extern "C" __rust_maybe_catch_panic()
. As such it’s up to ∞x faster. It’s also
aesthetically a little prettier (I think).
Example
Consider this motivating example:
enum States {
A(String),
B(String),
}
impl States {
fn poll(&mut self) {
// error[E0507]: cannot move out of borrowed content
*self = match *self {
// ^^^^^ cannot move out of borrowed content
States::A(a) => States::B(a),
States::B(a) => States::A(a),
};
}
}
Depending on context this can be quite tricky to work around. With this crate, however:
enum States {
A(String),
B(String),
}
impl States {
fn poll(&mut self) {
replace_with_or_abort(self, |self_| match self_ {
States::A(a) => States::B(a),
States::B(a) => States::A(a),
});
}
}
Huzzah!
Functions
Default::default()
on panic.